The field in which the use of the method according to the invention will be described in great detail is that of primary pumps of a nuclear reactor cooled by water under pressure. An example of such a pump currently in use is described with reference to FIGS. 1 and 2.
FIG. 1 shows a pump 100—called a primary pump—normally used in nuclear reactors cooled by water under pressure; primary pump 100 serves to put into circulation, in the cooling circuit of the nuclear reactor, referred to as the primary circuit, the cooling water of the nuclear reactor under pressure. Pump 100 comprises, in particular:
a pump body 101, connected by means of a first tube 102 to a first pipe of the primary circuit, and by a second tube 103 to a second pipe of the nuclear reactor; pump 100 puts the water into circulation between these two tubes, establishing a certain pressure;
a wheel 104, driven by an electric motor 105;
a drive shaft 106, having a lower section 110, called a pup shaft, this lower section being itself capable of being considered as a drive shaft, secured to wheel 104, and an upper section, called the motor shaft, connected by means of a coupling 107, called the intermediate shaft, to the rotor of electric motor 105; drive shaft 106, more particularly pump shaft 110, is generally cylindrical, although it includes, in an axial direction 109, of a succession of sections slightly different in diameter, thereby allowing the installation and housing of different elements, in particular pump wheel 104 or a protective thermal ring; the differences in the existing diameter, however, are sufficiently low to qualify the drive shaft as a generally cylindrical shaft;
an assembly of means of cooling, rotary guidance of shaft 106 and sealing arranged on the periphery of drive shaft 106; these means include, in particular, a thermal barrier 108, including a network of tubes through which a cooling fluid passes, and arranged around a lower section of the lower end of pump shaft 110;
a plurality of assembly means, in particular positively connected keys 112 driven into a recess arranged in the lower section of pump shaft 110, or rotary fixing 113 or stop pins inserted in recesses arranged in the drive shaft at the point where thermal barrier 108 is located.
The lower end of pump shaft 110 is caused to be immersed in water under pressure at a temperature of the order of 280° C. (degrees Celsius). Moreover, in normal operation, cold water, at a temperature lower than 55° C., is injected at thermal barrier 108 to form a barrage between the primary circuit and means of sealing the shaft. Under these conditions there is a transition zone 114 between the drive shaft and thermal barrier subject to major variations in temperature due to the mixture of water from the primary circuit at 280° C. and the injection water at 55° C.; the corresponding section of the shaft is therefore subject to major thermal stresses that are likely to result in the occurrence of cracks or a crazing phenomena in this section of the shaft, in if thermal rings are added. Cracks of thermal origin are multiple and form a network. The deepest cracks have a preferably axial orientation.
In addition to the cracks of thermal origin cracks of a mechanical origin are also observed: the latter appear either in the changes of cross-section or in the holes of stop pin 113 blocking the rotation of parts secured to drive shaft 106, or in the keyways, for example those of keys 112. They have a preferably transversal orientation. There are generally no multiple cracks, except where multiple pin holes are drilled. Moreover, drive shaft 106 is subject to risks of torsional fracture at the changes of cross-section found along the shaft.
Therefore the defects sought, whether they result from fatigue of mechanical or thermal origin, or even both, commence on the surface and spread progressively toward the inside of the part. They are mainly located in the lower section of the primary pump, a section subject to substantial temperature gradients.
In the state of the art different solutions have been proposed for detecting the occurrence of such defects in the lower section of pump shaft 110. In particular, several test methods have been proposed involving the use of ultrasonic transducers. Such transducers transmit ultrasonic beams in the shaft; these beams are reflected mainly at the location of the above-mentioned defects of the shaft; they are then received by the ultrasonic transducers that are coupled to an assembly of signal processing means for their interpretation and hence clearest possible detection of the existence and positioning of the said defects. In all the test methods there is an intermediate shaft that connects the pump shaft to the motor shaft, thereby providing access to an upper end of the pump shaft.
A first family of solutions resides in the use of transversal ultrasonic waves. Such an example of a solution is shown in FIG. 3, in which a drive shaft comparable to pump shaft 110 is tested. In this method an ultrasonic transducer—or sensor—is positioned at a gearing point 300, in contact with pump shaft 110; depending on the position of the sensor, different points to be tested may be reached, possibly after one or more reflections on the surface of the drive shaft, the beam being capable of taking different trajectories, different examples of which are denoted by references 301, 302, 303 and 304 respectively, so that each is able to reach a point to be tested. The sensors used are in most cases parallelepipedic mono-element transducers provided with a ferrule that enables the beam to deflect by an angle of up to approx. 75° to the axis of the shaft.
The advantage of this method is that it enables the defects to be located vertically and angularly: a rotary movement of the transducer about the drive shaft enables a circumference of the shaft, determined vertically by suitable axial positioning of the transducer, to be scanned. However, the transversal transducers must be used as close as possible to the zone to be tested; they must be arranged in positions that are lower than the directly accessible positions of the pump shaft; it is then necessary to dismantle the pump shaft, which complicates the test operation, particularly in the case of nuclear primary pumps. Such transducers also suffer from the disadvantaging of having a large beam opening, i.e. a low resolution, revealing only relatively major transversal defects. The axial defects arising from thermal fatigue are not detectable.
Moreover, it is necessary to use a ferrule for positioning the transducer in a given orientation, and hence obtain a desired angle of incidence, which complicates the test operation even more. Furthermore, the presence of thermal rings prevents the transducer from being positioned at the corresponding level of the shaft. Finally, it may be necessary, for testing the lowest points, to use several reflections of the ultrasonic beam, which involves a major loss of energy and thereby limiting the resolution of the return information. Therefore the longer the shaft the more difficult it is to apply the method.
A second family of solutions resides in the use of longitudinal waves. These test methods make use of transducers which transmit a divergent ultrasonic beam 400, shown in FIG. 4, whose width, at a great distance, i.e. at approximately three metres, is contained within the diameter of pump shaft 110. These solutions make use of transducers of the type with cylindrical mono-elements, annular mono-elements, or even annular transducers with two elements or with two groups of separate elements, with one element or a group of elements used for transmission, and one element or a group of elements used for reception.
These tests allow the detection of large transversal defects and their location vertically in the shaft. However, it is not possible to position them precisely and angularly on the periphery of the shaft. Transversal cracks of a mechanical origin, with a small reflecting surface, are difficult to detect since the resolution is dependent on the ratio of the area of the defect to the area of the beam in the same plane.
Moreover, axial defects are not detectable since the energy transmitted is never sufficient to be diffracted in sufficient quantity to the transducers through the upper end of axial defects.
Testing with this type of transducer may be improved by adding acoustic lenses, which improve the resolution and location of defects on the periphery but which suffer from the disadvantage of having a given focus, corresponding to a single distance, which involves having as many sensors as different heights to be examined. Moreover, axial defects remain non-detectable by these improved transducers.
Finally it should be noted that the operation which has just been described corresponds to a theoretical principle of use inherent in the technology of ultrasonic transducers in homogeneous and isotropic environments. Now forged shafts, and in particular shafts in austenitic or austenoferritic stainless steel, which are present mainly in the primary pumps of nuclear reactors, have a heterogeneity zone 500, shown in FIG. 5, with a metallurgical structure; these heterogeneities are found mainly in the thickest sections, and they vary from one shaft to another according to chemical composition, the forging range and the range of heat treatments applied.
N the thick sections of these shafts the metallurgical structure is therefore disturbed and remains course, which results in considerable retrodiffusion of the ultrasonic waves. This peculiarity results in the disturbance and attenuation of the transmitting and receiving power of the traditional transducers described above. However, the metallurgical structure is fine on the periphery, where the ultrasonic transmission is satisfactory.
This situation is aggravated when use is made of transducers with transversal waves whose beam necessarily crosses this heterogeneity zone twice for short shafts, i.e. shafts shorter than 2 metres, and even more for long shafts, i.e. shafts up to 3.5 metres long, considering the outward and return trajectories of the ultrasonic beams.